Exception on /webhook [POST}

When I try to use the Start Training button from labelstudio front end running v1.13.1 and current ML backend (commit 9fb7f4a) I get an error
```
ml-backend-1 | File "/usr/local/lib/python3.8/site-packages/label_studio_ml/api.py", line 125, in webhook
ml-backend-1 | model = MODEL_CLASS(project_id, label_config=label_config)
ml-backend-1 | TypeError: __init__() takes 1 positional argument but 2 were given
```

Looking at api.py line 125, the call to MODEL_CLASS is different than every other similar call to MODEL_CLASS

(lines 64, 99). When I update line 125 as below it works and calls my custom fit method in the backend. Is there something I am doing wrong to make it work in its current configuration?
```
model = MODEL_CLASS(project_id=project_id,
label_config=label_config)
```